Investigation on the effect of Tb(dbm)3phen on the luminescent properties of Eu(dbm)3phen-containing mesoporous silica nanoparticles
Graphical abstract
Sensitization of the antenna effect (down-conversion of UV radiation to red light) by the presence of Tb(dbm)3phen in the cavities of mesoporous silica nanoparticles containing Eu(dbm)3phen.
Introduction
Lanthanide organic complexes have been widely studied for their interesting absorption and emission features and have been suggested for applications in many different fields such as probes for biological systems [1], [2], LED [3], OLEDs [4], laser materials [5], or spectral converters for solar cells [6], [7], [8], [9], [10], [11].
The present work was carried out having mainly the latter application in mind, where the developed material needs to meet special requirements, like transparency, broad band absorption, long-term stability. In fact, harnessing those regions of the solar spectrum which are only weakly, or not at all, converted into electricity by multicrystalline silicon solar cells is highly desirable, in order to increase efficiency and reach grid-parity. Down-shifters, which convert UV radiation into visible or near infra-red light, may serve this goal [8]; these materials need to efficiently absorb a broad spectral range between 300 nm and 450 nm and re-emit with a large Stokes shift in the region where the solar cells show a significantly better response. Lanthanide ions have sharp emission profiles with high internal efficiency, but their absorption is very small and it takes place only on very specific wavelengths; this may be appropriate for applications where excitation can be made by lasers, but not for solar spectrum conversion. However, if the lanthanide ion is conjugated with suitable organic ligands having an electronic structure which matches that of the lanthanide, the desired optical properties for this application may be obtained; in fact, the ligand acts as an antenna, efficiently absorbing in a broad spectral region (typically in the UV) and transferring this energy to the rare earth ion, which then emits in the visible. At the same time, in order to obtain a good quantum yield, the emitted radiation has not to be quenched by the chemical environment, or the matrix in which the ions are included, and/or by concentration quenching. Furthermore, the organic ligand may suffer from UV degradation or thermal instability.
In order to address such problems two synergic approaches have been tested in this paper. The first is that of hosting the complex molecules within the pores of properly synthesized mesoporous silica nanoparticles. The pores were chosen of suitable size in order to “dilute” the emitting ions, thus avoiding concentration quenching and at the same time protecting them from the environment. Such an approach has shown to improve luminescence in similar cases [12], [13], [14], [15]. Since the final goal is to obtain a layer transparent to visible light, the mesoporous silica must be in the form of nanoparticles to avoid light scattering; such nanoparticles can be then dispersed in the encapsulating layer of the solar cell. We show that this approach is able to host up to 23 wt% of a lanthanide complex before concentration quenching takes place, which means some orders of magnitude more than in solution.
The second approach is that of co-doping with two different lanthanide complexes, where not only the antenna, but also the second lanthanide ion can act as sensitizer for the first ion. To this purpose, the Tb3+/Eu3+ pair is well known, where Tb ions can efficiently transfer energy to Eu ions increasing the latter's emission in the red spectral region. In this situation UV light is absorbed by the ligand and then transferred to Eu ions, either directly or indirectly mediated by Tb. Optimizing the ratio between Tb- and Eu-complexes within the pores of silica nanoparticles, it is possible to avoid concentration quenching [16] and obtain high red emission intensity.
In the present paper the behaviour of silica mesoporous nanoparticles with different loadings of Eu-complex is investigated in details and the effect of adding a second lanthanide complex is studied.
Section snippets
Materials
Tetraethyl orthosilicate (TEOS, Aldrich, 98%), hexadecyl-trimethylammonium bromide (CTAB, Aldrich), europium trichloride hexahydrate (EuCl3·6H2O, Aldrich, 99.9%), terbium trichloride hexahydrate (TbCl3·6H2O, Aldrich, 99.9%), ammonium hydroxide solution (Fluka, 28 wt% in water), absolute ethanol (99.8%, Carlo Erba) were all used as received. Eu(dbm)3phen and Tb(dbm)3phen complexes (tris(dibenzoylmethane) mono(1,10-phenantroline) Ln(III)) were synthesized according to the procedure reported by
The complex
Fig. 1 shows the molecular sketches of the two complexes used in this paper. They are formed by one phenanthroline and three dibenzoylmethane ligands connected by dative bonds to the lanthanoid ion, Eu or Tb, respectively.
Fig. 2 shows absorption, excitation and emission spectra of the Eu(dbm)3phen complex in solution. Absorption takes place on two bands: one, centred at 270 nm, is attributed to the absorption by the phenanthroline ligand [16], and is of no interest for terrestrial solar
Conclusions
The inclusion of Eu(dbm)3Phen inside mesoporous silica nanoparticles, either alone or together with Tb(dbm)3Phen, has a positive effect on luminescence, since the emission increases linearly up to much higher Eu-loadings (23 wt%) than in other matrices before showing concentration quenching. Co-doping with Tb enhances the emission of the material, but reaches a limit for a total amount of the complex of about 10 wt%. The results could be interpreted by using a simple sphere of action model
Acknowledgements
Alessia Le Donne, University of Milano Bicocca, is acknowledged for fruitful discussions.
References (20)
- et al.
Sol. Energy Mater. Sol. Cells.
(2009) - et al.
Sol. Energy Mater Sol. Cells
(2007) - et al.
Opt. Mater.
(2011) - et al.
J. Solid State Chem.
(2010) - et al.
J. Photochem. Photobiol., A: Chem.
(2007) - et al.
Dalton Trans.
(2011) - et al.
Chem. Rev.
(2010) - et al.
Adv. Mater.
(1991) - et al.
Renew. Sustain. Energy Rev.
(2012) - et al.
J. Am. Chem. Soc.
(1993)
Cited by (17)
Synthesis of silica/rare-earth complex hybrid luminescence materials with cationic surfactant and their photophysical properties
2019, Journal of Physics and Chemistry of SolidsCitation Excerpt :However, RE complexes can not be attached to the silica directly due to their different polarity. Publications have demonstrated it can be resolved by synthesize mesoporous silica nanoparticles, non-covalent grafting into the surface of silica by a seeded growth method or link RE complexes to the silica by a covalent bond [27–29]. In our previous work, SiO2@SiO2:Eu(DBM)3phen was prepared successfully via a seeded grow method and encapsulating RE complexes into silica shell, found that the photostability has been increased a lot [30].
Ag nanoaggregates as efficient broadband sensitizers for Tb<sup>3+</sup> ions in silica-zirconia ion-exchanged sol-gel glasses and glass-ceramics
2018, Optical MaterialsCitation Excerpt :These last can provide a broadband and very efficient sensitization of the RE3+ central ion, but they have some limitations due to the chemical and physical stability of the organic antenna. This can be improved by incorporation in a silica scaffold [26,27], but of course an inorganic alternative approach would be desirable, especially for lighting and solar applications. In the last two decades, extended literature has been published on the broadband and efficient sensitization of Er3+ ions by silicon [28–34] or silver aggregates [35–39], with significant potentials in optical communications, demonstrating the role of multimers and nanoaggregates that act as energy-transfer centres to the RE3+ ions.
An investigation into nanohybrid states of europium (III) complex with hydroxyapatite nanocrystals
2018, Optical MaterialsCitation Excerpt :The rod-like morphologies and luminescence properties were clarified. Even though the introduction of Eu3+ ion into inorganic host nanospaces [21,32–34] and crystalline structures [20] has been reported, the suppression of the Eu3+ ion aggregation has not been achieved. We proposed a crystal growth mechanism where the interaction with EuTH induced the changes in interaction (e.g., charge-transfer [46]) from the central Eu3+ ion to the ligands as well as to the Ca2+ and phosphate ions in HA.
Synthesis of di-functional ligand and fluorescently labeling SiO<inf>2</inf> microspheres
2018, Optical MaterialsCitation Excerpt :The excitation spectrum of complexes was obtained monitoring the emission at the hypersensitive 5D0→7F2 transition at 610 nm. These excitation spectrum exhibited a broad band at 200–300 nm and a strong broad band at 350–420 nm (λmax = 394 nm), corresponding to the Eu-O change transfer (CT) process and π-π* transitions of the organic ligand, respectively [13,27,28]. The results suggest that the ligand work as “antenna” for the Eu3+ ion and the efficient energy transfer from DBM-Si to Eu3+ ion has occurred, augmenting the number of photons converted into visible region.